1. Field of the Invention
This invention relates to a process for the manufacture of iron and aluminum-free, highly concentrated, aqueous, acidic zinc chloride solutions resulting from the separation of the aluminum and the iron from mixed solutions of these elements.
2. Description of the Prior Art
At the present time, large quantities of technical zinc chloride are obtained from waste products. These include ammonium chloride slag, zinc ash, pickling solutions, zinc chloride-containing waste liquors from the cellulose industry, etc. The zinc content of these waste products fluctuates between 40 and 80%. After treatment in the conventional manner with hydrochloric acid, 30 to 50%, and usually about 40% zinc chloride-containing crude liquors are obtained. These frequently must be freed from impurities and especially extraneous metal ions, such as, iron and aluminum.
In the processes presently used, the bivalent iron is first oxidized to trivalent iron using suitable oxidizing agents, such as, hydrogen peroxide, chlorate or permanganate. Alkali hydroxides or carbonates are used for the subsequent separation of the trivalent iron and the aluminum portion in the form of hydroxides. However, in order to not introduce any extraneous ions, zinc oxides or basic zinc salts are preferred as precipitation or neutralization agents. At a pH of 3.5 to 4, iron and aluminum hydroxides are then precipitated on heating to 80.degree. to 100.degree. C. After decanting and filtering, the precipitated and washed deposits are isolated.
The isolation of the deposits is particularly difficult, and, at times, even practically impossible, if the zinc chloride-containing cooled liquor contains a relatively large amount of aluminum. The deposits formed by the precipitation occupy a large value. As a result, their filtration proceeds so slowly that isolation of the metal values is economically possible. In addition, a considerable amount of zinc chloride which cannot be washed out, is retained in these voluminous precipitates.
When isolating crude zinc chloride liquors with low aluminum contents, filtration outputs or rates which are practically justifiable are &lt;100 l/m.sup.2 /hour. In the absence of aluminum, filtration rates of &gt;200 l/m.sup.2 /hour are achieved.
In practice, zinc chloride liquors can be processed only if the weight ratio of the iron portion to the aluminum portion is of the order of 4:1. In the case of a lower iron: aluminum ratio, such as, 4:2, the filtration and washing of the highly voluminous precipitate which is formed is practically impossible. Accordingly, zinc raw materials could not previously be worked up by wet chemical means.